A semiconductor device is manufactured by repeating a lithography step for projecting and exposing a device pattern formed on a master (e.g., a reticle or mask) to a substrate (e.g., a wafer or glass substrate) coated with a photosensitive material and developing the device pattern. In such a manufacturing step, it is important to accurately align a device pattern (latent image) to be projected and exposed to a photosensitive material to a device pattern (patterned structure) already formed on a substrate.
An example of operation for aligning the pattern of a master to a pattern formed on a substrate is substrate alignment. In the manufacturing step of forming a semiconductor device on a wafer, such substrate alignment is called wafer alignment.
Wafer alignment is performed in accordance with the following procedure. First, a wafer is supplied to a lithography system having an exposure apparatus and a mechanical alignment apparatus. Coarse alignment is done by the mechanical alignment apparatus using an orientation flat or notch formed at the peripheral portion of the wafer. Then, the wafer is placed on the wafer chuck of the exposure apparatus by a wafer supply apparatus. The typical alignment accuracy by the mechanical alignment apparatus is about 20 μm.
Next, the positions of a plurality of alignment marks (position detection marks) formed on the wafer by a preceding step are detected using an alignment scope. The X- and Y-direction shifts and rotation component of the wafer and the magnification component of the shot array are calculated on the basis of the detection result. In exposing each shot, the wafer stage is driven on the basis of the calculation result. Accordingly, the pattern already formed in a shot is accurately aligned to the pattern projected onto the wafer through a projecting optical system. This scheme will be called global alignment. The accuracy of global alignment is 50 nm or less in an exposure apparatus for manufacturing, e.g., a 256-Mbit memory.
In recent years, a planarization technique by a polishing step called CMP (Chemical Mechanical Polishing) is often used. When CMP is executed, the layer on the alignment mark is polished. This degrades a mark signal or decreases stability. To prevent this, an alignment mark is often optimized in accordance with the process. This optimization is performed by forming a plurality of tentative alignment marks having different structures such as line widths, pitches, and three-dimensional patterns and selecting an optimum alignment mark. Normally, an optimum alignment mark is determined at the time of prototype formation. In a flexible manufacturing system, however, mass production sometimes starts without executing optimization. In this case, a plurality of alignment marks may enter the visual field of the alignment scope.
Additionally, in recent years, a method of forming a plurality of sets of alignment marks in one region (exposure region) is replacing a method of forming a set of X and Y alignment marks in one region. This aims at, e.g., correcting a deformation of the exposure region or increasing the measurement accuracy by an averaging effect obtained by measuring a plurality of alignment marks. For such purposes, the accuracy must be increased by ensuring the span between the alignment marks is as wide as possible. More specifically, four alignment marks are formed at the four corners of each exposure region. Also, with this background, a plurality of alignment marks may enter the visual field (the field of view) of an alignment scope at a high probability.
Furthermore, as the number of steps increases recently along with an increase in complexity of a device structure, the number of times of alignment mark formation increases.
More specifically, the number or layout density of alignment marks increases in recent years in accordance with various purposes or factors such as optimization of the alignment mark structure, improvement of measurement accuracy, and the increase in the number of steps. Accordingly, a plurality of alignment marks enter the visual field of an alignment scope. For this reason, the necessity for identifying or specifying a target alignment mark from a plurality of alignment marks in the visual field is increasing.